This invention relates to a process for preparing 2-(1-cyclohexenyl)-cyclohexanone (abbreviated and referred to hereinafter as CHNA) by condensation of a cyclohexanone. CHNA so produced can be dehydrogenized to ortho-phenyl-phenol, which is useful as a preservative.
The preparation of CHNA has been described in various patents and publications. According to German patent specification No. 857,960 CHNA can be obtained by passing cyclohexanone through an acid ion exchanger at 90.degree. C. and subjecting the resulting liquid to distillation under reduced pressure. Using this procedure the yields amount to only 80% of the yield theoretically possible, while a relatively large quantity of ion exchanger is needed. In Compt. rend. 236, 1571 (1953) Durr states that the condensation of cyclohexanone to CHNA at a temperature of 130.degree.-140.degree. C. takes place with only a very low yield, that is 20%, if the commercially available ion exchanger known as Amberlite IR-120 is used. Contrary to Durr's publication, Japanese Patent Publication No. 70-41,377 mentions for the relevant conversion using the same ion exchange resin Amberlite IR-120 and a temperature of 130.degree.-135.degree. C., a yield of 97.5% is obtained. French patent specification No. 2,186,456 in turn mentions as a substantial disadvantage of the process according to the Japanese Patent Publication that the optimum temperature range of 130.degree.-135.degree. C. mentioned in that publication is in fact higher than the maximum permissible working temperature of most cation exchangers, which in practice results in high production costs.
According to this French Patent publication the desired conversion is better accomplished using a lower temperature of 80.degree.-110.degree. C., a pressure of 60-200 millimeter Hg and, during the conversion, by separating off the water formed by evaporation. From the reaction mixture obtained the required product is then recovered by distillation under reduced pressure. A readily apparent disadvantage of this process is the required use of reduced pressure, which on a commercial scale is costly to install and operate. Even using the reduced pressure technique the yield achieved is no greater than approximately 90%.